In the coated glass industry, it is often desirable to apply one or more thin layers of coating material to glass to impart desired properties in the resulting coated glass. For example, infrared-reflective coatings (e.g., low-emissivity coatings) are commonly applied to glass sheets. Coatings of many different types are used to impart in coated substrates desired properties, such as particular levels of transmissivity, reflectivity, absorptivity, emissivity, shading performance, color, durability, hydrophilicity, hydrophobicity, and photoactivity.
A variety of coatings are applied to glass for use in architectural and automotive applications. These coatings are commonly applied using “in-line” vacuum coaters with magnetron sputtering sources. For example, cylindrical magnetrons are used in many sputter deposition methods. In these methods, a substrate is positioned in a vacuum chamber containing at least one cylindrical target. Cylindrical targets are well known in the present art and commonly take the form idealized in FIGS. 4A and 4B. The cylindrical target 180 comprises a backing tube 182 carrying a thick outer layer of sputterable target material 185. The backing tube 182 is typically a rigid, elongated tube of electrically-conductive material (e.g., metal), which may be coated with a relatively thin bonding layer 184. A stationary magnet assembly 170 is typically positioned within the interior cavity 188 of the rotatable target 180. This magnet assembly 170 confines plasma in the chamber to a region adjacent the target 180. The target 180 is commonly mounted in the chamber to a pair of opposed end blocks (cantilever end block systems are also known), with each end block being adapted to hold one of the ends 189 of the backing tube 182.
During sputtering, an electrical field is typically created between a cathode and an anode in the sputtering chamber. Commonly, the sputtering target functions as the cathode and at least one separate anode is provided in the chamber at a location spaced away from the target. For example, a separate bar or another electrically-conductive member may function as the anode. Gas is delivered to the chamber to facilitate producing a plasma (e.g., a glow discharge). Electrons accelerate in the electrical field, gaining enough energy to ionize the gas atoms and create the plasma. Positively-charged particles (e.g., ions) in the plasma are attracted to the cathodic target, bombarding it and causing particles (e.g., atoms) of the target material to be ejected from the target. As sputtering continues, more and more particles are emitted from the target, causing erosion of the layer of target material 185. Eventually, the useful target material is depleted and the target must be replaced.
Thus, the continuity of the sputtering process is limited by the amount of useful target material on the target. Generally, sputtering is only continued until the useful target material is consumed, at which point each consumed target is no longer used. Thus, the sputtering process must be shut down periodically and the consumed targets replaced. This prevents manufacturers from operating their astronomically expensive sputtering lines (e.g., costing many millions of dollars) on a continuous, uninterrupted basis.
The coatings (e.g., low-emissivity coatings) used for architectural and automotive applications commonly comprise metal film and transparent dielectric film. When depositing metal film, a metal target is typically sputtered in the presence of inert gas, such as argon. When depositing dielectric film, a metal target is commonly sputtered in the presence of a reactive gas (e.g., oxygen or nitrogen). Thus, a reaction product (e.g., a metal oxide or metal nitride) of the metal target material and the reactive gas is deposited on the substrate. In some cases, dielectric films are alternatively deposited by sputtering ceramic targets in substantially inert atmospheres (optionally containing some reactive gas).
Thus, in depositing low-emissivity coatings and many other types of coatings, each substrate is commonly passed through a series of connected sputtering chambers (i.e., a sputtering line), wherein some of the chambers are adapted for depositing metal films and others are adapted for depositing dielectric films. Unfortunately, the continuity of the sputtering process in both types of chambers is limited by the need to replace consumed targets. The continuity of sputtering in chambers adapted for depositing dielectric films is even more severely limited, as will now be discussed.
During sputtering, an undesirable contamination layer builds up on the walls and other interior surfaces of the sputtering chamber. Ejected particles of target material are deposited on the substrate and, unfortunately, on other exposed surfaces within the sputtering chamber (walls, anodes, shields, rollers, etc.). Over time, a layer of sputtered material (i.e., a “contamination layer” or an “overcoat”) builds up on the interior surfaces of the chamber. This is particularly problematic in chambers where dielectric films are deposited.
The contamination layer grows increasingly thick as sputtering is continued. As the thickness of this layer increases, its internal stress builds up until the point is reached where flakes begin to spall from the contamination layer. When this occurs, some of the spalling flakes can fall upon a freshly-deposited coating on the substrate, leaving inclusions or pinholes in the coating. This can be a problem in chambers where dielectric films are deposited, because dielectric films tend not to adhere to the interior chamber surfaces as well as metal films. This problem is exacerbated by the fact that a dielectric contamination layer is electrically nonconductive and can cause arcing, which can cause larger chunks to fall from the contamination layer onto the substrate.
Growth of an electrically nonconductive contamination layer on the anode(s) in a sputtering chamber can cause particular problems. Transparent dielectric films are commonly insulators, semiconductors, or other electrically nonconductive materials (e.g., Si3N4, SiO2, TiO2, ZnO, SnO2, and Al2O3). The accumulation of such materials on an anode in a sputtering chamber initiates a progressive slowing of the sputtering process, which can ultimately result in a shut-down of the process. This phenomenon is commonly referred to as the “vanishing” or “poisoned” anode problem. A nonconductive contamination layer on the anode inhibits and eventually prevents charge carriers from flowing between the anode to the cathode. This has the effect of first reducing and eventually stopping the sputtering process. This also results in the conductive area of the anode changing in size during sputtering, thus rendering the sputtering process more difficult to control and potentially leaving non-uniformities among deposited coatings.
Growth of a dielectric contamination layer can complicate the sputtering process in further respects. For example, the charged plasma can be repulsed from the contamination layer due to like polarity of plasma particles and areas of contamination. As this repulsion increases, the distribution of the plasma may change as it “searches” for a conductive outlet. Moreover, as the conductive areas in the chamber become randomly distributed, the uniformity of the plasma discharge can deteriorate, thereby slowing the sputtering process. This non-uniformity of the plasma discharge can complicate process control and reduce the quality and uniformity of deposited films.
Growth of a contamination layer on the gas distribution system in a sputtering chamber can also cause particular problems. For example, when enough contamination builds up on gas delivery ports (e.g., so as to prevent gas from being delivered freely into the chamber), the sputtering process can be slowed due to insufficient plasma generation. In particular, when enough contamination builds up on the gas delivery ports in reactive sputtering, there may be insufficient reaction between the sputtered material and the reactive gas. Thus, the deposited films may be more metallic than is desired. This can make it difficult to deposit films meeting desired product specifications. It can also complicate controlling the process stability and sputtering rate.
To combat the foregoing problems, manufacturers periodically shut down their coaters and clean the contamination layer off the interior surfaces of each chamber. This involves venting each chamber, carefully cleaning it (e.g., sandblasting, scraping, or otherwise manually removing the contamination layer), and then re-evacuating the chamber. Consumed targets are typically replaced while the chambers are open for cleaning. The chamber cleaning process takes a great deal of time and effort. It is estimated that manufacturers lose as much as 40% of their potential production time to chamber clean-ups and target change-outs. This down-time is extremely expensive given the staggering cost of industrial sputtering equipment. Thus, it can be appreciated that a continuous coating process would provide an outstanding boost in productivity.
The foregoing problems can be aggravated when manufacturers try to stretch the productive operating period of a coater. At the end of the productive period for a given chamber, the plasma discharge searches for uncontaminated areas in the chamber. To keep the plasma discharge active, manufacturers sometimes deliver extra gas into the chamber. This can be risky for manufacturers, as the extra gas can create a nucleation curtain or a clustering of sputtered material. As a result, electrical current can be directed to flow through the nucleation curtain into the gas distribution system seeking a path back to the power supply, thereby causing arcing that can melt the gas pipe, etc. In addition to necessitating the installation of new gas pipes, this can have adverse effects on the sputtering process.
Solutions have been proposed to some of the foregoing problems. One proposed solution is disclosed in U.S. Pat. No. 4,863,756, issued to Hartig et al., the entire contents of which are incorporated herein by reference. This Hartig patent describes methods and equipment for applying coating to a moving substrate. In this patent, gas is delivered to a deposition chamber and is converted to a plasma. One upwardly-oriented magnet assembly is used to create a magnetic trap that confines the plasma to a localized area above the moving substrate. Immediately above the magnet assembly is a plate-shaped electrode connected to a voltage source. The voltage source is either a direct-current source or a high-frequency source with a frequency of approximately 13.56 MHz. Two reels guide the substrate in a horizontal path of travel directly above the electrode. Above the path of substrate travel is a gas-delivery system that provides reaction gas. In operation, the substrate is conveyed over the electrode, the electrode converts the gas into plasma, the system of magnets holds the plasma adjacent the substrate, and the plasma creates a chemical reaction and/or decomposition by which coating is deposited on the substrate. Since the plasma is trapped adjacent the substrate, the conversion of reaction gas to coating occurs only in the immediate vicinity of the magnets (e.g., directly over the substrate). Thus, coating is deposited over the substrate, but not over interior chamber surfaces remote from the magnetic trap.
In another embodiment, Hartig discloses a rotatable guide roller that doubles as an electrode. In the interior of this guide roller, there is one stationary, upwardly-oriented magnet assembly that is used to create the magnetic trap. During operation, a continuous flexible substrate is passed over the guide roller as it is rotated. In this embodiment, any unwanted coating that accumulates on the electrode is spread over its large cylindrical surface.
This solution is well suited for coating substrates in reel-to-reel applications (e.g., thin metal film or thin insulating film supplied from a reel, coated, and collected on a wind-up reel). In such applications, the disclosed solution is useful for avoiding production stoppages for chamber cleaning. This solution also obviates the need for targets as source material. Thus, there are no targets or target change-outs, only continuous conversion of gas into solid coatings, and only on areas of the substrate within the magnetic trap. Unfortunately, this solution has significant limitations.
For example, this solution is effectively limited to coating continuous film-like substrates in reel-to-reel applications. In the case of glass, coating is commonly performed upon spaced-apart sheets, wherein gaps are left between adjacent sheets. These gaps may account for as much as 30% of the available load area on industrial sputtering lines (e.g., load factors of 70% are not uncommon). With the equipment of this Hartig patent, such gaps would expose the electrode to unwanted coating. This would make it necessary to clean the electrode periodically, thus defeating the goal of having a continuous coating process.
Further, this solution is not well suited for coating non-conductive substrates on commercial “in-line” coaters. The Hartig patent teaches use of a high frequency (HF) power supply to establish current flow through non-conductive substrates. Unfortunately, HF cathodes are only available in sizes that allow coating a width of up to about 48″, whereas large area substrates (e.g., glass for architectural or automotive applications) commonly exceed this width. Moreover, it is extremely difficult to match the output impedance of an HF power supply to the constantly changing impedance of a plasma. The mismatching of impedance creates arcing, which can be damaging to the substrate and chamber. Further, it is difficult to achieve uniform distribution of HF power along the cathode when impedance mismatching occurs. This can cause non-uniformity among different areas of the coating. Thus, it would not be practical to deposit coatings on non-conductive large area substrates using high frequency power supplies.
The Hartig patent also indicates that direct current (DC) cathodes can be used. Insofar as non-conductive substrates are concerned, a DC cathode would create a negative electric field on a non-conductive substrate. This negative electric field would make it extremely difficult to maintain stable plasma, which is necessary for uniform film deposition. Therefore, a DC power supply would not be desirable for coating non-conductive substrates using the equipment of the Hartig patent.
It would be desirable to provide methods and equipment for continuously coating substrates without the problems discussed above.